Diagnostic method for an exhaust aftertreatment system

ABSTRACT

A diagnostic method for testing malfunction of a Selective Catalytic Reduction (SCR) exhausts aftertreatment system of an internal combustion engine. The method measures and registers a first mean NOx sensor value for a normal mass flow level of a fluid reducing agent, then a second mean NOx sensor value related to a higher mass flow level is measured and registered. The two registered NOx sensor values are compared and it is determined if, on one hand, the first value is higher than the second value, or, on the other hand, the first value is lower than the second value. The cause of the malfunction may be found in the dosing system or in the SCR catalyst.

TECHNICAL FIELD

The present invention relates to the field of a diagnostic method fortesting malfunction of a Selective Catalytic Reduction (SCR)aftertreatment system for reduction of NOx in an exhaust gas passage ofan internal combustion engine.

BACKGROUND AND SUMMARY

Vehicles equipped with diesel or another lean burn engine offer thebenefit of increased fuel economy, however, control of nitrogen oxide(NOx) emissions from such engines is needed due to the high content ofoxygen in the exhaust gas. In this regard, Selective Catalytic Reduction(SCR) catalysts, in which NOx is continuously removed through activeinjection of a reductant, such as urea, into the exhaust gas mixtureentering the catalyst, are know to achieve high NOx conversionefficiency.

A typical lean burn exhaust gas aftertreatment system is described inU.S. Pat. No. 6,928,806 and includes a SCR catalyst for converting NOxin the engine exhaust gas mixture by means of injection of a reductantagent. NOx sensors upstream and downstream of the SCR are coupled in thepath of the exhaust gas entering and exiting the SCR catalyst. Theoutputs of these sensors are read by controller 12 and may be used todetermine the NOx conversion efficiency of the SCR.

The above described aftertreatment system enables detection of amalfunction in the NOx conversion system, however, the cause of themalfunction may be found in the urea-dosing system or in the SCRcatalyst. For example, the urea-dosing system may be clogged, or the SCRcatalyst may be deactivated.

Thus, there is a need of a diagnostic tool for diagnosis of SCR catalystsystems, to facilitate service and repair.

It is therefore desirable that the present invention provide a diagnosismethod which is capable of accurately diagnosing the cause ofmalfunction.

The aspect of the present invention resides in a diagnostic method fortesting malfunction of a Selective Catalytic Reduction (SCR)aftertreatment system for reduction of NOx in an exhaust gas passage ofan internal combustion engine, the SCR system comprising a dosing systemthat injects a fluid reducing agent into the exhausts upstream acatalyst reactor and a downstream side NOx sensor for supervising theNOx emission in the exhausts downstream the SCR reactor, the diagnosticmethod comprising the steps of: setting the torque range and the speedrange of the engine to a predetermined interval and adjusting the fluidagent mass flow to a level which is normal for this interval; measuringand registering a first mean NOx sensor value for the mass flow level,increasing the fluid agent mass flow to a level which is higher thannormal for the interval; measuring and registering a second mean NOxsensor value related to the higher mass flow level; and comparing thetwo registered NOx sensor values and determining if on one hand, thefirst value is higher than the second value, or on the other hand, thefirst value is lower than the second value.

The other features of this invention will become understood from thefollowing description with reference to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be described in greater detail below with referenceto embodiments shown in the accompanying drawings, in which

FIG. 1 diagrammatically illustrates an internal combustion engine withan aftertreatment system for utilizing the invention, and

FIG. 2 is a flowchart showing a malfunction diagnosis according to theinvention.

DETAILED DESCRIPTION

FIG. 1 shows a general configuration of an exhaust aftertreatment systemfor an internal combustion engine 10, including a first exhaust pipesegment 11 leading exhausts from the engine to a urea based SelectiveCatalyst Reduction (SCR) catalyst 12. The engine can for example be adiesel engine for a heavy duty vehicle. The SCR catalyst is connected toa clean-up catalyst reactor 13 via a second exhaust pipe segment 14. Athird exhaust pipe segment 15 leads the exhausts from the reactor 13 tothe atmosphere.

The SCR catalyst is, preferably, a base metal/zeolite formulation.Reductant, such as aqueous urea, is stored in a storage vessel (notshown) and delivered to a reductant delivery system 16 coupled to thepipe segment 11 upstream of SCR catalyst 12. The reductant is meteredout by a pump through a control valve and an injector, where both thepump and the valve is controlled by a controller 17, preferably amicroprocessor. A NOx sensor 18 is coupled to the pipe segment 15downstream the clean-up catalyst 13. The reductant metering is based oninput data, for example engine torque load, engine speed and output fromthe NOx sensor 18.

If and when a malfunction occurs in aftertreatment system, i.e. theregistered tail-pipe NOx level exceeds a desired limit, one needs todetermine what component in the system is malfunctioning. Firstly, it isappropriate to check that there is urea in the solution and that the NOxsensor is not faulty.

If it is established that urea is present and the NOx sensor isreliable, the malfunction can only result from either that the catalystis deactivated or that the urea-dosing system is clogged (or possiblyboth). It is in this situation that the diagnose method of the inventionis useful. Isolating the faulty component is of great value for serviceand repair. It is also possible that future on-board diagnosislegislation will require that the faulty component is isolated.

The details of the reduction system diagnosis according to the inventionare explained with reference to the flowchart in FIG. 2.

The situation is that the NOx sensor 18 downstream the SCR system hasindicated that tail-pipe NOx is increased. Before the diagnosis it hasalso be established that there is urea in the solution dosed by thedosing system and that the NOx sensor is indicating the correct value.

The purpose of the suggested method is to determine whether tail-pipeNOx is too high due to that the catalyst is deactivated or that theurea-dosing system is clogged and therefore injects too little urea. Aprerequisite for the method to give the correct result is that thecontrol of the urea mass flow is such that it yields a tail-pipe NOxthat is the minimum of: a) Desired tail-pipe NOx concentration b)Minimum tail-pipe NOx concentration with respect to present catalystactivity

In step S1 and step S2, requirements on engine torque and engine speedare set. Possibly, the load-point range is restricted, and, secondly andmost importantly, the load point must be roughly the same during thetime that the diagnosis is carried out. This is due to that the methoduses the N0x-sensor value and if the engine-out NOx concentrationdiffers strongly during the diagnosis, this will drown the variationthat is to be detected. (Thus, if the deviation from the initial loadpoint becomes too large, the procedure must be interrupted and restartedat the new load-point, etc.)

In step S3, the urea mass flow is kept at the normal value determined bythe urea control system. The mean NOx-sensor value downstream thecatalyst is measured. This value is here called “NOx-conc_normalUrea”.

In step S4, the urea mass flow is increased so that it is somewhathigher than the normal value for the present load point, and the meanNOx-sensor value is measured. This value is here called“NOx-conc_increasedUrea”.

Now, the two NOx-sensor signals are compared in step S5. Two outcomesare defined:

-   A) NOx-conc_normalUrea≧NOx-conc_increasedUrea→urea-dosing system is    clogged-   B) NOx-conc_normalUrea<NOx-conc_increasedUrea→SCR catalyst is    deactivated

When step S5 is true, the urea dosing system is clogged and the ureadosing is not sufficient according to step S8. Then, increasing the ureadosing will increase the conversion of NOx in the catalyst or, possibly,the clogging is so severe that the increase in dosing does not result inany extra injected urea at all. Both these cases are covered by A.

When step S5 is not true, it is the catalyst that is faulty according tostep S7 and the increase in urea mass flow will not increase theconversion of NOx, since the catalyst is already working at its maximumcapacity. Instead, the excess NH3 will be detected by the NOx sensor asan increased NOx concentration. This results from that the NOx sensor iscross sensitive to NH3. Moreover, the clean-up catalyst downstream theSCR catalyst oxidizes the NH3 to NOx before it reaches the NOx sensor18.

Alternatively a variable calculated from the N0 x-sensor concentrationis used instead of the NOx concentration (e.g. the NOx conversion).

One advantage with the method is that it compares the relative size oftwo measured values. It is thus not dependent on any absolute values.

The invention is not to be regarded as being limited to the illustrativeembodiments described above, but a number of variants and modificationsare possible within the scope of the following patent claims. Forexample, the aftertreatment system of FIG. 1 may include a particulatefilter.

1. A diagnostic method for testing and diagnosing a cause of malfunctionof a Selective Catalytic Reduction (SCR) aftertreatment system forreduction of NOx in an exhaust gas passage of an internal combustionengine, the SCR system comprising a dosing system that injects a fluidreducing agent into exhaust upstream from a catalyst reactor and adownstream side NOx sensor for supervising NOx emission in the exhaustdownstream from the reactor, the diagnostic method comprising the stepsof: setting a torque range and a speed range of the engine to apredetermined interval and adjusting a fluid agent mass flow to a levelwhich is normal for the interval, measuring and registering a mean NOxsensor value for the mass flow, increasing the mass flow to a levelwhich is higher than normal for the interval, measuring and registeringanother mean NOx sensor value related to the increased mass flow,comparing the two registered NOx sensor values and determining if, onone hand, the NOx sensor value for normal mass flow is higher than theNOx sensor value for increased mass flow, or, on the other hand, the NOxsensor value for normal mass flow is lower than the NOx sensor value forincreased mass flow.
 2. A method according to claim 1, comprising theadditional step of controlling the value for normal mass flow to beequal to the value for increased mass flow.
 3. A method according toclaim 1, comprising the additional step of determining a type ofinjected fluid agent.
 4. A method according to claim 1, comprising theadditional step of determining that the NOx sensor operates correctly.5. A method according to claim 1, wherein a clean-up catalyst ispositioned in the aftertreatment system between the SCR catalyst and theNOx sensor.
 6. A method according to claim 2, comprising the additionalstep of determining a type of injected fluid agent.
 7. A methodaccording to claim 2, comprising the additional step of determining thatthe NOx sensor operates correctly.
 8. A method according to claim 3,comprising the additional step of determining that the NOx sensoroperates correctly.
 9. A method according to claim 2, wherein a clean-upcatalyst is positioned in the aftertreatment system between the SCRcatalyst and the NOx sensor.
 10. A method according to claim 3, whereina clean-up catalyst is positioned in the aftertreatment system betweenthe SCR catalyst and the NOx sensor.
 11. A method according to claim 4,wherein a clean-up catalyst is positioned in the aftertreatment systembetween the SCR catalyst and the NOx sensor.
 12. A method according toclaim 1, comprising providing a malfunction indication if the NOx sensorvalue for normal mass flow is higher than the NOx sensor value forincreased mass flow.